1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copy machine, and more particularly to an electrophotographic image forming apparatus.
2. Description of the Related Art
Such an electrophotographic image forming apparatus obtains a color image by a method in which a recording image (toner image) formed on a photosensitive drum as an image bearing member by charging, exposing and developing is transferred to a recording material repeatedly in plural processes to form a multi-color superposed image on the recording material. However, in the color image forming apparatus, a phenomenon in which an unwanted white gap appears between adjacent images of different colors may occur. The reason for this is as follows. In an electrostatic latent image that is formed in a condition in which a drum surface potential sharply changes, for example, an image edge portion is formed on a photosensitive drum, in the case where this portion is developed by a development apparatus, a visible image may be formed thinner than the electrostatic latent image originally formed on the photosensitive drum. Hereinafter, this phenomenon is referred to as a “white gap”. When a single color image is formed, any thinned image causes no problem because there is no adjacent color. However, when a color image is formed in a state in which for example, the image having a cyan color band is tried to be adjacent to a black color band, the cyan color band and the black color band in the image are respectively formed with thinner bands than those to be. Consequently, there is a problem to cause a gap between the cyan color and the black color in the final image formed on the recording material. FIG. 15A depicts the detail of the white gap according to a conventional technique and illustrates the electric field between a developing roller 226 and a photosensitive drum 222. In a portion of a visible image, the thinning of the visible image that causes a white gap occurs because the electric field is convoluted in an edge portion of an electrostatic latent image in an electrostatic portion formed on the photosensitive drum.
In order to solve this problem, there has been known a method of preventing image thinning by causing a light emitting element of a laser scanner to emit light slightly enough to prevent excessive toner adhesion in a non-print region (non-toner image forming region) (image background portion) in an entire printable region. Hereinafter, this method is referred to as background exposure. However, the purpose of the background exposure is not limited to preventing the white gap. For example, as disclosed in Japanese Patent Application Laid-Open No. 2003-323012, the background exposure is also performed as a reverse fog measure for a large potential difference (back contrast) between a developing bias potential and a primary charge bias. In addition, as disclosed in Japanese Patent Application Laid-Open No. 2000-131899, the background exposure is also performed as a measure for preventing air discharge (toner scattering) occurring in a transfer nip portion by reducing transfer potential contrast. In other words, the background exposure in the present description is not limited to the preventive measure for a specific purpose.
Here, examples of the preventive measure against the background exposure include a method of changing the duty ratio of a pulse wave, referred to as a PWM (Pulse Width Modulation). The method is to cause the light emitting element of the laser scanner to emit light with a pulse width corresponding to a slight light emission in a non-print region in synchronism with an image clock as a fixed frequency. Here, in the case where almost all print regions are white, the light emitting element slightly emits light over the entire print region so that fixed thin pulses occurs over the entire print region as a drive signal of the light emitting element of the laser scanner. As a result, there is a problem in that unnecessary radiation wave having a frequency corresponding to a pulse cycle of slight light emission greatly occurs. One solution of this problem is disclosed in Japanese Patent Application Laid-Open No. 2003-312050. According to Japanese Patent Application Laid-Open No. 2003-312050, the pulse width corresponding to a laser emission time period is randomly modulated and background exposure is performed based on the modulated pulse width to reduce unnecessary radiation wave.
Although the conventional technique as disclosed in Japanese Patent Application Laid-Open No. 2003-312050 can be expected to exert a protective effect against noise having a frequency higher than the image clock, it may not be sufficient for noise having a frequency band of the image clock. FIG. 15B describes the relation between an exposure pattern (emission pattern) and an electric field intensity distribution of unnecessary radiation noise. In FIG. 15B, the width A denotes a laser emission time period width (second) to drive the light emitting element of the laser scanner, and the width B denotes a total laser emission period (second) of the emission time period width and the non-emission time period width of the light emitting element of the laser scanner. Here, consideration is given to an electric field intensity distribution of unnecessary radiation when the light emitting element of the laser scanner is driven with the laser emission time period width A and the laser emission period B. Unnecessary radiation noise due to the laser emission time period width A occurs at a frequency of (1/A)×n [MHz] (n is a positive integer). Unnecessary radiation noise due to the laser emission period B occurs at a frequency of (1/B)×n [MHz] (n is a positive integer). Note that noise in a frequency band of the image clock refers to noise having a frequency illustrated by the width B in FIG. 15B. In light of this, unnecessary radiation measure members such as a shield, a filter and a core can be provided to reduce noise. However, an addition of an unnecessary radiation measure member may lead to an increase in cost, mounting area/volume, and weight. Thus, a simple measure is required to reduce noise in the frequency band of the image clock.
In view of this, it is an object of the present invention to solve at least one of the above and other problems. It is an object of the present invention to reduce an electric field intensity of an electromagnetic wave occurring as unnecessary radiation by means of a simple configuration.